Return the number of bytes requested by the server that have not yet
been written.
It is possible to write more than this amount, up to the stream's
buffer_attr.maxlength bytes. This is usually not desirable, though, as
it would increase stream latency to be higher than requested
(buffer_attr.tlength).
Return the number of bytes requested by the server that have not yet been written.
It is possible to write more than this amount, up to the stream's buffer_attr.maxlength bytes. This is usually not desirable, though, as it would increase stream latency to be higher than requested (buffer_attr.tlength).
(size_t) -1 is returned on error.